1. Field of the Invention
The invention relates to composite films having surface contact characteristics useful for temporary attachment to article surfaces during processing of such articles at high temperatures without significant increase in the force of attachment. Films of the invention facilitate device manufacturing operations associated with electronic microdevice assembly.
2. Description of the Related Art
Assembly of electronic microdevices is complex and includes a variety of physical and chemical processing steps which are performed upon smaller and smaller circuits. In fabricating, for example, a printed circuit supporting integrated circuit chips, it is necessary to protect or mask certain portions of the printed circuit while performing required processing steps such as the application of photoresist, soldering or encapsulating to a circuit which is on the order of a few millimeters in total size.
A masking material, typically an adhesive masking tape, limits application of fluid treatments to designated areas of the printed circuit. Most adhesive-type masking tapes, whilst fulfilling the requirement of confining treatments to desired areas, have the potential to cause problems after completion of the step requiring their use. This is primarily due to the difficulty in removing the tape. A tacky adhesive, used for tape attachment, typically has more affinity for the surface to which it is adhered than it has cohesive characteristics, especially when the process step requires heat, which tends to increase adhesion. At the time of removal, the adhesive frequently has insufficient cohesive strength for clean removal from the surface, resulting in contamination of the printed circuit surface. This transfer of adhesive to the surface of the printed circuit produces an area of contamination capable of capturing particulates or chemical components that could prevent the circuit assembly from functioning effectively. It is highly desirable for an adhesive in electronic component masking to separate from the protected surface without leaving any residual adhesive. This is extremely difficult to accomplish considering that formulation of adhesives primarily focuses on the need for measurable tack or stickiness when applying the adhesive. Even crosslinked adhesives, with higher cohesive properties, are subject to some adhesive transfer due to their tendency to flow, especially if part of the circuit assembly process includes a high temperature step.
Adhesive tapes are disclosed for use as masking tapes in electronic processing methods in JP 4251931 where a tape consisting of a substrate film layer composed of polypropylene or poly(ethylene terephthalate) having an adhesive layer coated thereon is used to protect the front side of a semiconductor wafer from acid treatment.
In JP 7216334 a film composition containing a polymer with at least 20% 2-acryloyloxyethyl succinic acid is used as a support for a protective film or tape to masking electronic parts.
Other references, including JP 6340848, JP 6184521, JP 3140380, JP 63227334, JP 57167254 and JP 74013630, disclose protective films which prevent surface damage to planar structures made from a variety of materials with emphasis on glass, metal and plastic plates. These protective films cover the total area of the underlying plate and remain in attachment with the surface of the plate until removal of the film. There is no disclosure or evidence to show that these films seal the gap between the film and a plate surface to prevent ingress of fluids as is required of protective masking films suitable for electronic circuit processing. Also neither clean removal nor high temperature performance appear to be requirements for films used to protect polished surfaces of plates.
U.S. Pat. No. 4,737,559 discloses a crosslinked adhesive which is controlled through the addition of a free radically polymerizable photocrosslinker to an alkyl acrylate and methacrylate for adhesion and to skin and removal therefrom.
Effective processing and prevention of surface contamination introduced by masking films used in processing electronic circuits requires a material which will provide good temporary adhesion or attachment to the circuit as well as a residue-free separation from selected areas of a printed circuit, even after exposure to heat.
The current invention provides a temporary attachment material in the form of a composite, tack-free film having an affinity for attachment to printed circuit surfaces. Because the surface which contacts the printed circuit is part of a continuous film, free of the fluid characteristics of an adhesive, it releases cleanly, and no residue remains on the circuit after removal of the composite film.